In a radial compressor, a fluid (e.g. air) is first of all drawn in axially via a compressor wheel connected upstream of a diffuser and is accelerated and pre-compressed in the compressor wheel. In this process, energy in the form of pressure, temperature, and kinetic energy is supplied to the fluid. At the outlet of the compressor wheel, high flow rates prevail. The accelerated, pre-compressed air leaves the compressor wheel tangentially in the direction of the diffuser. In the diffuser, the kinetic energy of the accelerated air is converted into pressure. This takes place by deceleration of the flow in the diffuser. Through radial expansion, the flow cross-section of the diffuser is enlarged. The fluid is thus decelerated and pressure is built up. In order to achieve pressure ratios that are as high as possible in a turbocharger with a radial compressor, the diffusers that are used therein can be provided with a blading. An example of a bladed diffuser is shown by German Patent Application Publication No. 102008044505 A1 (the entire disclosure of which is incorporated by reference herein). The diffusers with blading that are known from the prior art are generally configured as radial parallel-walled diffusers with blading, as shown for example in U.S. Pat. No. 4,131,389 (the entire disclosure of which is incorporated by reference herein). In order to achieve a greater compressor efficiency at a given overall pressure ratio, the flow in the diffuser can be decelerated more greatly. The flow rates in the spiral are reduced as a result, with the result that the wall friction losses decrease and the efficiency of the compressor stage is improved. The use of diffusers with radial side-wall divergence allows greater deceleration with the same overall length compared with parallel-walled diffusers.
However, the deceleration or pressure increase that is achievable in the diffuser by geometric variation for a given operating point is limited, since flow instabilities arise in the diffuser on account of boundary layer separation in the event of excessive deceleration. The limits of the stable operating range of the diffuser thus determine the position of the surge line of the compressor in the compressor characteristic map. If, instead of a parallel-walled diffuser, a diffuser with side-wall divergence is used—such a diffuser is described for example in PCT International Publication No. WO 2012/116880 A1 (the entire disclosure of which is incorporated by reference herein)—although the efficiency increases with identical compressor pressure ratios, at the same time the surge line moves toward greater mass flows at a given compressor pressure ratio compared with the compressor with a parallel-walled diffuser. This effect is not desired. The width of the compressor characteristic map is thereby reduced, and the usability of the compressor stage for applications in a turbocharger is thereby limited. One solution is to fluidically connect a diffuser duct portion of a bladed diffuser to an annular duct via pressure equalizing openings in order to allow pressure equalization between individual diffuser passages of the diffuser which are formed by adjacent diffuser blades. However, in this solution using pressure equalizing openings, the problem of the annular duct and/or the individual pressure equalizing openings becoming clogged can arise (e.g. on account of residues and deposits from compressor cleaning or by particles which are found in oil-containing intake air). This has a negative effect on the surge line of the compressor and, in extreme cases, can result in an engine connected to the diffuser no longer being able to be operated.